1. Field of the Invention
The present invention relates to printers. More particularly, the present invention relates to a paper feeding unit in a printer that reduces curling of the recording stock and maintains a gap between the printer head and the print media.
2. Discussion of Prior Art
Ink-jet printers, facsimiles, copy machines and like apparatuses employ printer heads. Referring to FIGS. 7-10, many printers include a head cartridge 1 containing ink used to form characters on print media, a carriage 2 that moves the head cartridge 1 along the print area, and a driving unit (not illustrated). The printer also includes a paper cassette 3 holding paper P, pickup rollers 4 for picking up paper, a paper sensor (not illustrated), conveyance rollers 5, aligning the leading edge of the paper and conveying it, and a registration sensor 6 controlling the conveyance rollers 5 and aligning the leading edge of the paper. A guide 7 conveys the paper, and a paper delivery roller 8 discharges the paper.
Once a print command is received by the printer from a host computer, the control unit of the printer directs the drive unit to operate. The pickup rollers 4 pick up the paper P one sheet at a time from the paper cassette 3 and deliver the sheets to the conveyance rollers 5 along the guide 7.
The registration sensor 6 converts a distance to the entrance to the conveyance rollers into time, and adds paper to be conveyed according to a value obtained by the conversion to let the paper be curled between the pickup rollers 4 and the conveyance rollers 5, aligning its leading edge. The conveyance rollers 5 deliver the paper to the printer front zone below a nozzle 1a of the cartridge 1. The cartridge 1 on the carriage 2 is translated according to a signal produced from a head driver, depositing ink particles on the paper to form characters thereon. Once the one-line printing is completed, the conveyance rollers 5 transfer the paper by a given length to complete the overall printing job, and the paper is output through the paper delivery roller 8, thus finishing one-cycle printing.
In the conventional ink-jet printer several problems may occur frequently when the paper P, picked up by the pickup rollers 4, is conveyed to a printing zone S. Referring to FIG. 9, a conveyance zone y exists between line k and the printing zone S. The paper, transferred by the conveyance force of the conveyance rollers 5, passes through the conveyance zone y into the printing zone S where characters are formed on the paper P by the cartridge 1. The characteristics of the paper P vary with the humidity and temperature of the room in which the printer is situated. The paper P must be used in a room with proper humidity and temperature to ensure good print quality. Improper temperature and humidity causes a paper curling problem in the conveyance zone y. The result of the paper curling problem is particularly acute after the paper moves into the printing zone y. In a room having improper temperature and humidity, the paper curls toward the printer or image-transferring head, which causes distorted images, smearing and other problems. Conventional printers typically do not have any mechanisms for preventing the paper curling problem. Therefore, printing sometimes is carried out on the paper having a portion elevated beyond a printing interval formed between the nozzle 1a of the cartridge 1 and a guide frame 11. During printing, some areas of the paper P gets close to the nozzle 1a of the cartridge 1, the interval between the paper P and the nozzle 1a being not constantly maintained, thus making the print quality inferior. In the worst case, the curled paper directly contacts the nozzle 1a of the cartridge 1 to make the nozzle 1a of the cartridge 1 unclean and deteriorating the overall print quality. In addition, the nozzle 1a of the cartridge 1 must be cleaned frequently. The paper curling problem also makes it difficult to introduce the leading edge of the paper between the paper delivery rollers 8, thus resulting in a paper jam.
The above demonstrates a need for a mechanism that reduces the potential image transfer and paper feed problems associated with paper curl.
U.S. Pat. No. 5,564,847 for Media Handling in an Ink-jet Printer Having Guide Ribs issued to Patrick et al., and also U.S. Pat. No. 5,527,123 for Media Handling in an Ink-jet Printer issued to Jackson et al., include an upper print media guide 118 having an extending portion 119 that terminates in a lower support edge 122 which contacts and supports paper 110 from above. A lower print media guide 126, downstream from the upper medium guide, includes an upwardly extending support edge 130. Although the medium 110 is shown maintaining a uniform spacing between it and a printer head 103, between the two dashed lines of FIG. 3, the lower support edge 122 and the upper support edge 130 actually urge the medium 110 to assume a humped, or non uniform, print area relative to the head 103.
U.S. Pat. No. 5,420,621 for Double Star Wheel for Post-printing Media Control in Inkjet Printing issued to Richtsmeier et al., referring to FIG. 1, includes a main drive roller 15 with idler roller 16 cooperating to maintain tension in the sheet 12. The device also includes a starwheel-type pinch wheel 28 which cooperates with an output driver roller 26 to maintain tension in the sheet and to control the position of the sheet relative to the printer head 22.